There have been many attempts to achieve noise reduction by the use of active control and there are many publications relating to these previous systems. Most have dealt with the control of a single source of unwanted vibration, while others have treated multiple sources as a single, more complicated source. For example, in the control of cabin noise in propeller driven aircraft, the individual propellers are separate sources which are physically separate and, since the engine speeds are not identical, not completely coherent. However, previous methods to control aircraft cabin noise, have considered the vibrating cabin walls as a single, distributed source. Vide Warnaka, G. E. & Zalas, J. M. "Active Attenuation of noise in a closed structure." UK Patent 2,132,053: Groves, H. W. "Noise Suppression." UK Patent 2,126,837: Nelson, P. A. & Elliott, S. J. "Improvements in or relating to active noise reduction." UK Patent 2,149,614.
Most active control systems use a measure of the residual vibration at a point where reduction is desired to adjust the actuator drives vide Chaplin, G. "Active attenuation of recurring sounds." UK Patent 1,577,322. This measure will be corrupted by noise, due either to extraneous "background" vibration or to electronic noise. The level of this noise will limit the performance of the control system. If the source of vibration to be controlled is periodic, or almost periodic, in nature then the process of synchronous averaging can be used to reject noise which is not harmonically related to the vibration source. A signal from the vibration source is needed to synchronize the averaging process. This may be a signal from a tachometer or an applied voltage or similar. If the noise is random in nature then the ratio of the signal power to the noise power is increased in proportion to the number of averages. However, if the noise is from another periodic source of not identical speed, the noise to signal ratio is an oscillating function of the averaging time. Reference is made to FIG. A of the drawings, in which the envelope decays only slowly and scales with the best period of the two periodic sources. Conventional active control means then adopt one of two strategies.
The first is to average for a sufficiently long time, compared with the beat period, so that the envelope of the noise to signal ratio has decayed to a small enough value.
The second is to treat the vibration as if it were coming from a single source. This requires the system to adapt on a time-scale short compared with the beat period.
The first method cannot be used if the signal, that is the unwanted vibration, is not constant for long enough. The second method fails if the two fundamental frequencies drift apart producing a very rapid beating.